<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    *{margin: 0;}
    html,body,#parent{
      height: 100%;
    }
    #parent{
      display: flex;
    }
    #left{
      flex: 1;
      background:yellow;
    }
    #right{
      flex: 2;
      background: pink;
      overflow-y: scroll;
    }
  </style>
</head>
<body>
  <div id="parent">
    <div id="left">
      嘿嘿嘿
    </div>
    <div id="right">
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
      <h1>3333</h1>
    </div>
  </div>
</body>
</html>